It's not possible to simply specify that:

"compression is the most important thing" or

"glow plugs are the most important thing".


The issue is simply the temperature of the inside of the cylinder when fuel is injected. If you have an engine with very good compression, you can probably start it without glow plugs down to maybe 32 degrees or so. Then, with the use of glow plugs, you can start it down to -10 or so, if the fuel does not gel.

If you have an engine with some miles on it, the situation is simply shifted. You can easily start it without glow plugs at maybe 75 degrees or so. Then, with the use of glow plugs, you can start it down to +10 or so.

Compression certainly helps with starting, but, a brand new engine from the factory will not start at 10 degrees without the use of glow plugs.